Where concrete has failed to prevent highly radioactive water pouring into the sea, workers at Japan's Fukushima Daiichi nuclear power plant have shifted hope of plugging the leaks to an absorbent polymer mixed with sawdust and shredded newspaper that expands 50-fold when in contact with water.

Although officials conceded the polymer had made little impact so far, they will wait until Monday before deciding whether to abandon it. "We were hoping the polymers would function like diapers, but we have yet to see a visible effect," said Hidehiko Nishiyama, spokesman for Japan's nuclear safety agency.

Officials separately has warned that the nuclear crisis could drag on for months, the first time that they have offered a timescale. Goshi Hosono, an aide to the prime minister, Naoto Kan, said everything possible was being done to contain leaks, which have contaminated the environment and food and water supplies, necessitated mass evacuations, and fomented fear as far away as Tokyo, 150 miles to the south. "We have not escaped from a crisis situation, but it is somewhat stabilised," said Hosono. Asked how long it would take to bring the overheating reactors under control, he said: "I think several months would be one target." The timeframe was echoed by Nishiyama, who said: "It will take a few months until we finally get things under control and have a better idea about the future. We'll face a crucial turning point within the next few months, but that is not the end of it."

The admission that there is no end yet in sight to the world's worst nuclear accident since Chernobyl came after the recovery of two bodies inside the plant, staff killed when it was struck by the tsunami on 11 March. After almost three weeks of radiation, they had to be decontaminated before they could be handed to relatives. Kazhiko Kokubo, 24, and Yoshiki Terashima, 21, died from multiple head injuries, reports said. Theirs were the first confirmed deaths at the Daiichi plant.

"It pains us to have lost these two young workers, who were trying to protect the power plant during the earthquake and tsunami," said Tsunehisa Katsumata, the chairman of Tokyo Electric Power [Tepco]

The task facing 600 workers, troops and firefighters at the plant was underlined at the weekend by the discovery of a 20cm crack in a concrete pit connected to No 2 reactor. The leak generated radiation of 1,000 millisieverts an hour in the air inside the reactor, while Tepco said it could also be behind the seepage of radioactive iodine into the sea, sending contamination to 4,000 times the legal limit.

Experts say that beyond the vicinity of the plant, there is minimal risk to human health in other parts of Japan or overseas. After failing to seal the crack by pouring in concrete, workers have tried to use the polymer mix to close it.

Highly radioactive water has flooded the basement of the No 2 reactor and a connecting underground trench. The water in the cracked pit is thought to have come from partially melted fuel rods in the reactor's core.

In their battle to cool overheated reactors and prevent a dangerous meltdown of highly radioactive fuel rods, workers have doused reactors with huge quantities of seawater. The contaminated runoff, however, has prevented technicians and engineers from getting closer to the reactors to make proper repairs.

Fresh water is being pumped into No 1, 2 and 3 reactors using external power, Nishiyama said, adding that the remaining three reactors were considered stable.

More than 12,000 people are known to have died in the earthquake and tsunami, while 15,472 are missing, according to police. More than 163,000 people are still living in evacuation shelters.